A number of modules are included the Perl
distribution. These are described below, and all end in .pm. You may also discover files in the library directory that end in either .pl or .ph. These are old libraries supplied so that old programs that use them still
run. The
.pl files will all eventually be converted into standard modules, and the .ph files made by h2ph will probably end up as extension modules made by h2xs. (Some .ph values may already be available through the
POSIX module.) The pl2pm file in the distribution may help in your conversion, but it's just a
mechanical process and therefore far from bulletproof.

They work somewhat like pragmas in that they tend to affect the compilation
of your program, and thus will usually work well only when used within a
use, or no. Most of these are locally scoped, so an inner
BLOCK may countermand any of these by saying:

no integer;
no strict 'refs';

which lasts until the end of that
BLOCK.

Unlike the pragmas that effect the $^H hints variable, the use
vars and use subs declarations are not BLOCK-scoped. They allow you to predeclare a variables
or subroutines within a particular
file rather than just a block. Such declarations are effective for the entire
file for which they were declared. You cannot rescind them with no vars or no subs.

Extension modules are written in
C (or a mix of Perl and
C) and may be statically linked or in general are dynamically loaded into Perl if and when you need them. Supported extension modules include the Socket, Fcntl, and
POSIX modules.

Many popular
C extension modules do not come bundled (at least, not completely) due to their sizes, volatility, or simply lack of time for adequate testing and configuration across the multitude of platforms on which Perl was beta-tested. You are encouraged to look for them in
archie(1L), the Perl
FAQ or Meta-FAQ, the
WWW page, and even with their authors before randomly posting asking for their present condition and disposition.

CPAN stands for the Comprehensive Perl Archive
Network. This is a globally replicated collection of all known Perl
materials, including hundreds of unbundled modules. Here are the major
categories of modules:

(The following section is borrowed directly from Tim Bunce's modules file, available at your nearest
CPAN site.)

Perl implements a class using a package, but the presence of a package doesn't imply the presence of a class.
A package is just a namespace.
A class is a package that provides subroutines that can be used as methods.
A method is just a subroutine that expects, as its first argument, either the name of a package (for ``static'' methods), or a reference to something (for ``virtual'' methods).

A module is a file that (by convention) provides a class of the same name (sans the .pm), plus an import method in that class that can be called to fetch exported symbols. This module may implement some of its methods by loading dynamic
C or
C++ objects, but that should be totally transparent to the user of the module. Likewise, the module might set up an
AUTOLOAD function to slurp in subroutine definitions on demand, but this is also transparent. Only the
.pm file is required to exist. See the perlsub manpage, the perltoot manpage, and the AutoLoader manpage for details about the
AUTOLOAD mechanism.

If so, please try to reuse the existing modules either in whole or by inheriting useful features into a new class. If this is not practical try to get together with the module authors to work on extending or enhancing the functionality of the existing modules.
A perfect example is the plethora of packages in perl4 for dealing with command line options.

If you are writing a module to expand an already existing set of modules,
please coordinate with the author of the package. It helps if you follow
the same naming scheme and module interaction scheme as the original
author.

Pass arrays as references so more parameters can be added later (it's also
faster). Convert functions into methods where appropriate. Split large
methods into smaller more flexible ones. Inherit methods from other modules
if appropriate.

Avoid class name tests like: die "Invalid" unless ref $ref eq 'FOO'. Generally you can delete the ``eq 'FOO''' part with no harm at all. Let the objects look after themselves!
Generally, avoid hard-wired class names as far as possible.

Avoid $r->Class::func() where using @ISA=qw(... Class ...) and
$r->func() would work (see the perlbot manpage for more details).

Use autosplit so little used or newly added functions won't be a burden to programs that don't use them. Add test functions to the module after
__END__ either using AutoSplit or by saying:

eval join('',<main::DATA>) || die $@ unless caller();

Does your module pass the 'empty subclass' test? If you say ``@SUBCLASS::ISA = qw(YOURCLASS);'' your applications should be able to use
SUBCLASS in exactly the same way as
YOURCLASS. For example, does your application still work if you change:
$obj = new YOURCLASS;
into: $obj = new SUBCLASS; ?

Avoid keeping any state information in your packages. It makes it difficult
for multiple other packages to use yours. Keep state information in
objects.

Always use -w. Try to use strict; (or use strict qw(...);). Remember that you can add no strict qw(...); to individual blocks of code that need less strictness. Always use -w. Always use -w! Follow the guidelines in the
perlstyle(1) manual.

Coding style is a matter of personal taste. Many people evolve their style
over several years as they learn what helps them write and maintain good
code. Here's one set of assorted suggestions that seem to be widely used by
experienced developers:

Use underscores to separate words. It is generally easier to read $var_names_like_this than $VarNamesLikeThis, especially for non-native speakers of English. It's also a simple rule that works consistently with
VAR_NAMES_LIKE_THIS.

Package/Module names are an exception to this rule. Perl informally
reserves lowercase module names for 'pragma' modules like integer and
strict. Other modules normally begin with a capital letter and use mixed
case with no underscores (need to be short and portable).

You may find it helpful to use letter case to indicate the scope or nature
of a variable. For example:

Exports pollute the namespace of the module user. If you must export try to
use @EXPORT_OK in preference to @EXPORT and avoid
short or common names to reduce the risk of name clashes.

Generally anything not exported is still accessible from outside the module
using the ModuleName::item_name (or $blessed_ref->method) syntax. By convention you can use a leading underscore on names to
indicate informally that they are 'internal' and not for public use.

(It is actually possible to get private functions by saying:
my $subref = sub { ... }; &$subref;. But there's no way to call that directly as a method, because a method
must have a name in the symbol table.)

As a general rule, if the module is trying to be object oriented then
export nothing. If it's just a collection of functions then
@EXPORT_OK anything but use @EXPORT with caution.

Select a name for the module.

This name should be as descriptive, accurate, and complete as possible.
Avoid any risk of ambiguity. Always try to use two or more whole words.
Generally the name should reflect what is special about what the module
does rather than how it does it. Please use nested module names to group
informally or categorize a module. There should be a very good reason for a
module not to have a nested name. Module names should begin with a capital
letter.

Having 57 modules all called Sort will not make life easy for anyone
(though having 23 called Sort::Quick is only marginally better :-). Imagine
someone trying to install your module alongside many others. If in any
doubt ask for suggestions in comp.lang.perl.misc.

If you are developing a suite of related modules/classes it's good practice
to use nested classes with a common prefix as this will avoid namespace
clashes. For example: Xyz::Control, Xyz::View, Xyz::Model etc. Use the
modules in this list as a naming guide.

If adding a new module to a set, follow the original author's standards for
naming modules and the interface to methods in those modules.

To be portable each component of a module name should be limited to 11 characters. If it might be used on
MS-DOS then try to ensure each is unique in the first 8 characters. Nested modules make this easier.

How do you know that you've made the right decisions? Have you picked an
interface design that will cause problems later? Have you picked the most
appropriate name? Do you have any questions?

The best way to know for sure, and pick up many helpful suggestions, is to
ask someone who knows. Comp.lang.perl.misc is read by just about all the
people who develop modules and it's the best place to ask.

All you need to do is post a short summary of the module, its purpose and interfaces.
A few lines on each of the main methods is probably enough. (If you post the whole module it might be ignored by busy people - generally the very people you want to read it!)

Don't worry about posting if you can't say when the module will be ready -
just say so in the message. It might be worth inviting others to help you,
they may be able to complete it for you!

It's well known that software developers usually fully document the software they write. If, however, the world is in urgent need of your software and there is not enough time to write the full documentation please at least provide a
README file containing:

To be fully compatible with the Exporter and MakeMaker modules you should store your module's version number in a non-my package variable called
$VERSION. This should be a floating point number with at least two digits after the decimal (i.e., hundredths, e.g,
$VERSION = "0.01"). Don't use a ``1.3.2'' style version. See Exporter.pm in Perl5.001m or
later for details.

It may be handy to add a function or method to retrieve the number. Use the
number in announcements and archive file names when releasing the module
(ModuleName-1.02.tar.Z). See perldoc ExtUtils::MakeMaker.pm for details.

It's good idea to post an announcement of the availability of your module
(or the module itself if small) to the comp.lang.perl.announce Usenet
newsgroup. This will at least ensure very wide once-off distribution.

If possible you should place the module into a major ftp archive and
include details of its location in your announcement.

Some notes about ftp archives: Please use a long descriptive file name that
includes the version number. Most incoming directories will not be
readable/listable, i.e., you won't be able to see your file after uploading
it. Remember to send your email notification message as soon as possible
after uploading else your file may get deleted automatically. Allow time
for the file to be processed and/or check the file has been processed
before announcing its location.

If it ain't broke, don't fix it! Perl 4 library scripts should continue to
work with no problems. You may need to make some minor changes (like
escaping non-array @'s in double quoted strings) but there is no need to
convert a .pl file into a Module for just that.

Perl does not enforce private and public parts of its modules as you may have been used to in other languages like
C++, Ada, or Modula-17. Perl doesn't have an infatuation with enforced privacy. It would prefer that you stayed out of its living room because you weren't invited, not because it has a shotgun.

The module and its user have a contract, part of which is common law, and part of which is ``written''. Part of the common law contract is that a module doesn't pollute any namespace it wasn't asked to. The written contract for the module
(A.K.A. documentation) may make other provisions. But then you know when you
use RedefineTheWorld that you're redefining the world and willing to take the consequences.

Went to join the gridlock to see it
Held an eclipse party
Watched a live feed
I cn"t see tge kwubosd to amswr thus
I tried to see it, but 8000 miles of rock got in the way
What eclipse?
Wanted to see it, but they wouldn't reschedule it
Read the book instead